When it comes to ensuring sufficient personal and plant protection during the operation of electrical power grid systems the insulation resistance is of special importance. If the insulation resistance drops below a predefined value protection against direct or indirect contact with the electrical plant is diminished; in addition leakage currents may occur leading to malfunctioning of electrical devices or to a costly interruption in the operation. Protection against fire is another reason why plant operators and the insurance industry have an interest in maintaining the plant in perfect technical condition as regards its insulation resistance.
It is therefore necessary to constantly monitor the insulation resistance in electrical plants. One active measuring process for measuring the insulation resistance has proven to be that of introducing an AC voltage as a measuring signal because it is easy to couple to the supply network via a transformer, provided the power supply network is grounded.
However, with processes based on AC voltage measurements, it has to be borne in mind that apart from the insulation resistance which is regarded as a purely ohmic resistance component, the capacitive component of the complex-valued network leakage impedance is another important factor. In particular in spatially expanded networks the network leakage capacity may increase to such an extent that determina-tion of the insulation resistance would be distorted by capacitive leakage currents. For a predefined measuring (AC-) voltage these capacitive leakage currents are dependent upon the capacitive conductance which again increases as the frequency increases. It is therefore desirable, on the one hand, to keep the measuring frequency as low as possible in order to counter the high network leakage capacity for expanded line assemblies, and on the other, minimising the capacitive conductance which increases proportionally with the frequency.
Furthermore, a low measuring frequency leads to a lower current load on the protective conductor if network leakage capacities are large. This, in turn, means a reduced load on the protective conductor from functional requirements thereby substantially precluding endangering the protective concept of a “protective conductor”. This as well promotes acceptance of a new device technology.
Selecting a suitable, i.e. a minimum measuring frequency, is therefore of special importance. A low measuring frequency of the impressing measuring signal permits monitoring of major grounded power supply networks, and any high-frequency interferences which may occur, for example from frequency converters, can be filtered out due to the greater frequency gap using less expensive filter circuits.
For example it is known from the applicant's published patent application DE 103 55 086 A1 to determine the insulation resistance by introducing a rectangular-shaped common mode voltage signal against ground, wherein feeding-in of the generator signal is preferably effected via a transformer. The measuring frequency results from the base frequency contained in the rectangular oscillation over time and is separated by means of filtering from the other higher-frequency signal components present in the line network. For a network frequency of 50 Hz it is proposed to use a measuring frequency of 175 Hz for the generator signal to be fed.
With this method which proposes supplying the generator signal by means of a transformer it has proved to be disadvantageous that there are limits as to how far the measuring frequency range can be extended towards the low frequencies by using a transformer. Since the induced voltage on the secondary side is proportional to the temporal change in magnetic induction the induced voltage also drops for a decreasing base frequency of the fed-in signal. In order to compensate for this decrease in voltage the number of windings and/or the core cross-section of the transformer could be increased by the same amount. This, however, would mean the use of disproportionately more expensive and larger transformers which in addition would be uneconomical with regard to power requirements. In order to achieve a voltage amplitude on the secondary side for measuring-signal frequencies, which lie distinctly below 100 Hz and that is sufficiently large for ensuring a reliable use of the process, the transformers required would be so expensive that corresponding terminal prices for products in line with the market would not be realistic in the intended user environment. According to the state of the art and based on economic considerations the measuring frequency is limited to values above approximately 100 Hz when using transformers for the signal supply. In conclusion it can therefore be said that the problem lies not in generating a low-frequency generator signal on the primary side, but in inductively transmitting or coupling it in a product-specific manner to the current supply network.